1. Field of the Invention
The present invention relates to pre-press proofs and photosensitive media, particularly photosensitive media for use in generating color surprint pre-press proofs, particularly where low dot-gain is desired.
2. Background of the Art
In color reproduction, it is often necessary to verify the color accuracy of separation negatives and/or positives used in the printing process. This can be accomplished by producing a four or more color proof. There are presently two main types of color proofs used by printers: overlay and surprint proofs. An example of a negative-acting overlay proof is described in U.S. Pat. No. 3,136,637. That proofing method is based upon exposing and developing each presensitized color sheet separately and then superimposing the sheet to yield a multicolor composite. This method offers a very simple way of combining two or more colors in registration. However, it has several inherent disadvantages. The multiplicity of plastic sheets causes incident light to internally reflect. This reflection imparts a gloss, a color hue shift, and a distortion of the image which is not truly representative of printed copy. U.S. Pat. No. 4,634,652 and U.S. patent application Ser. No. 06/928,598 filed Nov. 14, 1986, describe improved overlay systems using more transparent substrates. However, an optical distortion of the image and subsequent color hue shift still remains due to the large spacing between the colored images.
A surprint proof is generated by successively superimposing different colored layers upon each other on a single sheet. The surprint color proofing materials are composed of two primary types: (1) photosensitive precolored sheets and (2) photosensitive colorless sheets whose latent image may be toned with pigments, inks or dyes (also referred to as a "post-colored" proofing material). Some examples of post-colored proofing materials may be found in U.S. Pat. No. 3,060,024; U.S. Pat. No. 3,634,087 and DE 3,336,431. In these systems, the image is weak and often requires a protective layer. The resultant color hue is also very user-dependent due to dependency upon color density control and lot-to-lot variation of the colorants.
Some examples of precolored proofing materials include constructions based upon both diazo and photopolymers as the photosensitive element. In U.S. Pat. No. 4,482,625 and U.S. Pat. No. 4,304,836, two different precolored photopolymer constructions are described which are imaged prior to lamination to the substrate or an intermediate sheet. A similar system is described in GB 2,144,867 A which utilizes precolored films sensitized with diazo-based polymers. These systems require the imaged films to be laminated in registry which is very dependent upon laminating speed, laminating temperature, and dimensional stability of the film and substrate. The accuracy of the registration is limited to small format proofs. This type of system also makes it very difficult to produce several proofs on a single large sheet (scatter proofing).
U.S. Pat. No. 3,671,236; U.S. Pat. No. 4,260,673; U.S. Pat. No. 4,650,738, U.S. Pat. No. 4,656,114; U.S. Pat. No. 4,659,642 and EPO 115,899 A3 describe diazo-sensitized precolored constructions which are laminated to the substrate prior to imaging. Each of these systems contain barrier and/or adhesive layers which remain in the final proof, lying between each image layer. The existence of these additional clear layers causes an optical enlargement of the halftone dots, better known as optical dot gain. The optical gain in these proofing systems gives a reasonable reproduction of the tonal curves for printing presses having dot gains in the 20-30% range. The different dot gains can be obtained by increasing the coating weights of the barrier and/or adhesive layers. U.S. Pat. No. 4,262,071 describes the incorporation of a spacer layer in order to increase the optical dot gain. All of these methods are effective for achieving the higher dot gains. However, it is very difficult to achieve the dot gains in the 8-20% range.
One approach to reducing the dot gain of a proof involves the use of a special base paper coated with a TiO.sub.2 dispersion. EPO 305,599 describes a base paper of this type. However, the clear spacer layers (constituted by the adhesive of the medium) of the proofing films used with these papers must be as thin as possible. This is difficult to control.
EPO 243,932 describes an improved positive working color proofing film and process which claims to be able to reproduce dot gains in this lower range. This system is based upon toning a photopolymer image. Being a toning process, it suffers from a lack of reproducibility of the density and lot-to-lot color variance of the toners. The system described in EPO 243,932 patent application also incorporates optical brighteners which causes metamerism in the final proof.
U.S. patent application Ser. No. 07/185,971, filed on Apr. 25, 1988, in the name of A. K. Musser, describes a positive-acting presensitized colored sheet. To achieve low dot gain, a thin, 1.0 to 3.2g/m.sup.2, adhesive layer is used. This construction provides optical dot gains in 40% tints of 14% to 22% on 3M Commercial Base Paper, 3M Company, St. Paul, Minn. and 13% to 20% on 3M Low Gain Commercial Base Paper, 3M Company, St. Paul, MN. To obtain optimum quality in the final proof made with this construction, high laminator roll temperatures and slow roll speeds are required. At less stringent conditions, the laminations of the second-down and subsequent color layers can show bubbles in the underlying tints, especially the 50% and higher tints. These air bubbles provide a more matte appearance in the shadow areas and cause an apparent shift in color hue in these areas. When higher lamination temperatures are used to reduce or eliminate the bubbles, the dimensions of the paper base are reduced with each lamination, giving rise to misregistration of the colored images.